The invention relates to a method for the further drying and sterilization of partially dewatered sheet-like sewage sludge having a solid matter content of preferably 15 to 35%.
Various methods are known in connection with sewage sludge according to which the sludge is spread onto an endless sieve belt and successively lead through, for example, a first drainage zone for draining the free standing water, a second draining zone with continuously increasing compression, a further drying zone, and a "milling" zone in which additional water is removed from the sludge cake by shearing forces (German PS No. 2,351,097, German AS No. 2,522,505). After passing through the various drainage zones the sludge cake has a solid matter content of an average of 15 to 35% and a maximum of 40%. The sewage sludge drained according to this process no longer resembles a paste, but rather a plurality of sheets or small clumps, but is still incapable of being scattered as a crumbly or granular material. Furthermore, in many cases sterilization of the drained sludge is necessary.
To decrease water content, thus increasing solid matter content, and to simultaneously sterilize it, it has been proposed to mill or mix quicklime with the sewage sludge containing preferably from 15 to 35% solid matter.
This type of milling or mixing is in the opinion of persons skilled in the art necessary so that the entire sludge material--including the inner portions--can react chemically with the quicklime. To perform the milling, for example, a plough share (twin screw) mixer or a double wave mixer can be used. Although this procedure does result in a further decrease of liquid content, the mixing or kneading of lime and sewage sludge is quite expensive, because the necessary apparatus is very expensive. This type of mixing has the further disadvantage that the sewage sludge to be removed from the apparatus is in the form of a paste so that further expensive method steps (among others intermediate storage for several days) are necessary to make the product scatterable.